[ATAT]

My issue is this.. I thought this was a source issue, but apparently not. Doing listening comparisons with a friend's M^3 I noticed that the PPA v2 has a loud, audible static hiss at loud volumes.. with or without source plugged in.

I think this is a grounding or wireing issue, but since I'm not so sure, I'd like to ask first.

My configuration is this..

Buffer Bias ~ 20 mA.. measures 42-45 mV across output resistors (would a higher bias translate to less noise?)

Class-A Biasing with 1mA

Gain - 5

Resistors ..

R11/8WATT 4.32KOHMS 1%
R21/8WATT 1MOHMS 1%
R3/R111/8WATT 1KOHMS 1%
R41/8WATT 10KOHMS 1%
R51/8WATT 3.3KOHMS 1%
R61/8WATT 470KOHMS 1%
R8/R23/R331/8WATT 10OHMS 1%
R101/8WATT 750OHMS 1%
R131/8WATT 100OHMS 1%
R21/22/31/321/10WATT 220OHMS 0.1%
R24/342.2ohms 5% Tol

Opamps - 637s on L/R , 627 on middle

Wire, thin 24 AWG Silver plated copper wire..

Suggestions?

Thanks very much..

 

[tangent]

Quote:

Originally Posted by ATAT would a higher bias translate to less noise?

No.

Quote:

Suggestions?

Do everything in the Preliminaries section of this article, and report the results.

 

[ATAT]

Quote:

If the hiss level goes down as you turn the amp's volume down, the source is probably just noisy. If the hiss goes away when you unplug the source from the amp, there's the culprit. You can try decreasing the amp's gain or adding output resistors to attack this problem, but ultimately the best solution is to get a better source. If the hiss level doesn't change as you adjust the volume control, the amp is probably generating the noise. Either lower the amp's gain or add a small output resistor. The gain should be set so that your normal listening volume is about at the 3 o'clock position with your least efficient heapdhones and your weakest source. If your normal listening position is lower than that, the amp has too much gain and you're just amplifying natural noise along with the signal.

Okay. Something strange.. I thought this was source independent since when I unplugged it I heard the "noise". strange thing is, when I switch rooms, I no longer hear the noise. I think it may be that the wire is picking up some kind of interference?

So is the gain of 5 already low, or should I go and lower it a bit more?

(I just realized some of my solder joints are not very clean... would a bad joint lead to distortion?.. and I should go clean that soon...

 

[n_maher]

Quote:

Originally Posted by ATAT Okay. Something strange.. I thought this was source independent since when I unplugged it I heard the "noise". strange thing is, when I switch rooms, I no longer hear the noise. I think it may be that the wire is picking up some kind of interference?

Is the amp in an enclosure? If not, every wire in the amp is probably acting as an antennae. Even if the amp is cased up it could be picking up static with no source plugged in. I know this is confusing, but when a source is plugged in, but not playing do you get the noise?

Quote:

So is the gain of 5 already low, or should I go and lower it a bit more?

Quote:

Opamps - 637s on L/R , 627 on middle

What strikes me as odd is that you're allegedly running 637s, w/ a gain of 5. This is definitely a BAD idea. The 637 is rated as stable at gains greater than 5 and I personally wouldn't run it less than 8 or 10. All of your problems may be linked to this. I would certainly address this before trouble shooting any further.

Quote:

(I just realized some of my solder joints are not very clean... would a bad joint lead to distortion?.. and I should go clean that soon...

I doubt it, but a cold joint might cause noise. I double solder just about everything these days. Time consuming yes, but generally helps to avoid wondering about if a joint is good or not.

HTH,

Nate

 

[ATAT]

Mmm thanks N Maher.. to answer you question, it's in a case. But I have excessively long wires for my power section .. (whoops) . I think my I/c may be acting as a antennae? (cheap ratshack)

Static is there if I have the RCA end of the IC is in. As soon as I take that out, static stops. If i take out the mini-plug end of the IC, the noise changes but it's still there...

Yes, noise is there when no music is playing from the source

Okay. just to check the op-amp idea.. swapping to AD843s..

[oh and I installed the caps to prevent the opamps from oscillating..]

 

[n_maher]

Quote:

Originally Posted by ATAT Mmm thanks N Maher.. to answer you question, it's in a case. But I have excessively long wires for my power section .. (whoops) . I think my I/c may be acting as a antennae? (cheap ratshack)

Any chance of posting a picture? I had noise problems with a Millett that I built that were a combination of crappy wiring from my power supply (STEPS) to the amp in a shared case and also a bad grounding scheme. I'd be surprise to learn that an IC was the source of any significant noise, but I guess it's not impossible.

Nate

 

[ATAT]

I'm slightly embarrased at how bad the wireing is but eh. As the japanese say, Shame is the only way to really learn =p..

Side ... wires go behind cap to the output jack

Back side... leftmost side is battery cable.. the switch has two twined cable.. the power jack is the quagmire of wires that looks really tangled.. I should fix it. Yes the RCA only has one side grounded.. (I think it doesnt matter since RCA is single-ended right?) yes my stripping job was really bad on the input wires.

Overview. Forgot to take out the polycarb sheet on top .. whoops

 

[n_maher]

I'd say you'd be well served to clean up some of that wiring

. It may or may not be the source of your issues but can't hurt! I'd also connect the ground of the RCAs. FWIW the final wiring of my personal amps is going to be shielded four-conductor mic cable to try and help minimize noise as much as possible. We'll see if it makes any difference. But I can tell you that my PPA V2 is dead quiet and it doesn't have a case, it's just air wired. Sometimes it'll pick up some ambient noise but not much.

Finally, with a clear plastic top you're loosing any benefit of a metal chassis in the way of shielding.

Nate

 

[ATAT]

Mmm good call on the metal case.. didn't even think about that.. Okay. Time to clean up those wires =)

Thanks!

 

[Francis_Vaughan]

Chiming in, I would certainly clean up the wiring - it can't hurt. It has been known for a bad solder joint to be slightly diode like and detect RF too. The symptom is certainly RF pickup. Sometimes it sounds more fizzy than simple white noise, and sometimes you can hear the radio! But in the digital age much RF is carrying digital modulated information.

I'm guessing you have used the plastic ended case. You have only connected signal ground to one of the input grounds. Bad. You must do both.

With plastic ends and plastic top you only have half the case in metal. That isn't a great start to RF immunity.

Invest in some heat shrink - it helps make for much neater wiring.

This is my build. I try to be neat